Problem: A bag contains $7$ red jelly beans, $10$ green jelly beans, and $4$ blue jelly beans. If a jelly bean is randomly chosen, what is the probability that it is red?
Solution: There are $7 + 10 + 4 = 21$ jelly beans in the bag. There are $7$ red jelly beans. The probability is $ \frac{7}{21} = \dfrac{1}{3}$.